Thanks so much for your response.
Those were my thoughts exactly, it seemed to be some sort of character limit on the email, despite the fact that I've used much larger HTML scripts in the past... very odd.
I did try adding it into a new email template and the same issue arose. After investigating (staring at my screen for some time should I say), I realised that it was cutting the code off 2/3rds of the way through but still kept all the closing tags at the end of the email. I found the cut off point, copied the HTML from the downloaded code I had, and pasted it in from where it cut off. Miraculously this worked fine and fixed the issue... maybe it was just a little Marketo bug?!
Thank you again for your helpful response though!
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